1. Field of the Invention
The field of the invention is that of diversity signal reception. To be more precise, the invention concerns the equalization and the combination of signals received on the different channels of a diversity receiver. It applies with advantage to all selective and/or scrambled transmission media.
2. Description of the Prior Art
A preferred field of application of the invention is that of tropospheric transmission. A transmission medium of this kind is conventionally subject to Rayleigh fading (deep and fast, up to 15000 dB/s). It is also highly frequency selective, the coherence band of the transmission medium often being less than the transmitted bit rate.
Diversity reception is based on receiving the same source signal on a plurality of separate channels (at least two). Each of these channels is processed (equalized) independently. They are then combined (weighted sum). The combined signal is then conventionally processed to reconstruct the transmitted signal as reliably as possible.
In the case of tropospheric transmission, diversity is assured by simultaneous transmission of the same signal on as many carrier frequencies as there are channels It can also be achieved by spatial arrangement of a plurality of antennas near the receiver.
Diversity equalization-combination is subject to a number of problems, especially in tropospheric transmission.
At low signal/noise ratios, the equalizers tend to become desynchronized or to have difficulties with converging, because of decision errors in estimating the received information, which serves as a criterion for equalization.
It is very difficult to synchronize the equalizers before diversity combination. The differential propagation tire-delays between diversity channels rule out the "fixing" of a coefficient called the central coefficient. The resulting degree of freedom has to be compensated by another constraint for combination in phase.
To be more precise, on a highly selective transmission medium the various channels can have relative time-delays exceeding the transmitted signal period. On summation (combination), the various channels have to be synchronized. The computation is unstable and subject to catastrophic propagation (interaction of equalizers), a problem that it avoided or made less severe by using a slow algorithm.
There are three major classes of combiners, namely:
selection combiners: PA1 equal gain combiners: PA1 optimal combiners: PA1 either by the same information 17. In this case the equalizer itself acts as a weighting device. The desynchronization of the channels, or the poor quality of one channel, then causes the equalizer to diverge from the channel, which unbalances the combiner, which causes decision errors on the combiner channel, which causes the other equalizers to diverge, and so on. PA1 or by external information, such as an AGC voltage 19.sub.1 -19.sub.N, a noise measurement, etc. External information of this kind may be subject to caution primarily in the case of limited equalizer correction capacity or in the presence of a scrambler leading to AGC capture. It does not necessarily represent the quality of the channel. PA1 an equalizer controlled by means for computing equalization coefficients and delivering an equalized signal, PA1 a demodulator fed with the equalized signal and delivering a demodulated signal, and PA1 a weighting device receiving the demodulated signal and delivering a signal weighted in accordance with a weighting coefficient, the weighted signals being then combined by a diversity summing device delivering a combined signal and feeding a global estimator which delivers a global estimate of the transmitted source signal, PA1 a first channel error signal representative of a comparison between the demodulated signal and a channel estimate of the transmitted source signal, delivered by a channel estimator fed with the demodulated signal in the channel during an acquisition phase, and PA1 a second channel error signal representative of a comparison between the demodulated signal and the global estimate of the transmitted source signal during a normal operation phase, PA1 a first global error signal representative of a comparison between the combined signal and the global estimate of the source signal during the acquisition phase, and PA1 a second global error signal representative of a comparison between local reference information known to the receiver and corresponding received reference information periodically extracted from the global estimate during the normal operation phase. PA1 in phase 2 (normal operation): PA1 y.sub.k is the reference information known to the receiver; PA1 Z.sub.k is the reference information as contained in the combined signal, with: PA1 a first channel error signal representative of a comparison between the demodulated signal and a channel estimate of the transmitted source signal during an acquisition phase, and PA1 a second channel error signal representative of a comparison between the demodulated signal and the global estimate of the transmitted source signal during a normal operation phase, and wherein the weighting coefficients are computed in accordance with: PA1 a first global error signal representative of a comparison between the combined signal and the global estimate of the transmitted source signal during the acquisition phase, and PA1 a second global error signal representative of a comparison between local reference information known to the receiver and corresponding received reference information periodically extracted from the global estimate during the normal operation phase. PA1 the equalization coefficients of each channel are computed in accordance with the first channel error signal representative of a comparison between the demodulated signal and a channel estimate of the transmitted source signal, and PA1 the weighting coefficients are computed in accordance with the second global error signal representative of a comparison between local reference information known to the receiver and received reference information extracted periodically from the global estimate.
output=max (p.sub.1 ;p.sub.2 . . . ;p.sub.n) P.sub.i =1: power of signal on channel i S/N.sub.output =max ((S/N).sub.1) PA2 output=p.sub.1 +p.sub.2 + . . . p.sub.n PA2 if S.sub.1 =S.sub.2 =S.sub.n S/N.sub.output =(S/N).sub.i*n PA2 if S.sub.1 =0 for i&lt;&gt;k S/N.sub.output =(S/N).sub.k/n PA2 output=a.sub.1 p.sub.1 +a.sub.2 p.sub.2 + . . . +a.sub.n p.sub.n with a.sub.i =K*(S/N).sub.i S/N.sub.output =(S/N).sub.1 +(S/N).sub.2 + . . . (S/N).sub.n PA2 the equalizers converge from the error computed from the difference between their own decision and their own equalizer signal; PA2 the combiner converges from the combined error computed from its decision (estimate); PA2 the equalizers "track" on the basis of the error computed from the difference between the combined decision (therefore of best quality) and their own equalized signal; PA2 the combiner "tracks" on the basis of the error computed from the difference between the combined received reference signal and the local reference (reliable information). PA2 z.sub.k =a.sub.1 x.sub.k1 + . . . +a.sub.m x.sub.km, where:
where m is the diversity order or number of channels.
The selection combiner does not offer any combination gain. Its output is simply equal to the best input.
The equal gain combiner causes all channels to contribute, whether they enhance or degrade the combined signal.
The optimal combiner maximizes the signal/noise ratio. The problem is to evaluate this ratio on each channel.
FIG. 1 shows an equalizer-combiner of a type known in itself in which the combiner is an optimal combiner.
Each channel 11.sub.1 through 11.sub.n feeds an equalizer 12.sub.1 through 12.sub.n and then a demodulator 13.sub.1 through 13.sub.n and a weighting device 14.sub.1 through 14.sub.n. The weighted signals are combined by a summing device 15 which feeds a decision module 16 supplying an estimate 17 of the received signal for the remainder of the processing.
The equalization coefficients are determined by computation modules 18.sub.1 through 18.sub.n according to the estimate 17 of the received signal.
weighting before combination can be controlled:
When using a measurement of the received power (P=S+N; if N is fixed, then a=K*S), the problem (and the source of error) is to evaluate S in the measurement of P. The system is sensitive to the variation of N, to scrambling and to distortion (non-integrity of S).
In the case of noise measurement after AGC amplification (P=S+N=Constant and a=K/N) the noise measurement is done out of band, leading to errors. Moreover, the system is sensitive to the selectivity of the propagation medium, to scrambling, to distortion and to out-band spectrum pollution.
To overcome these problems so-called "reference directed" equalizers have been developed, relying on the insertion at the transmitter end of a reference sequence which is known at the receiver end. The coefficients of the equalizers and of the combiner are computed using only these reference symbols.
A device of this kind is shown in FIG. 2. The device 21 for analyzing the reference sequence drives the weighting devices 14.sub.1 through 14.sub.n and the modules 18.sub.1 through 18.sub.n for computing the equalization coefficients.
A disadvantage of this technique is that when the channel varies at a fast rate, the overhead due to the reference sequencer can become prohibitive, especially if the convergence algorithm is slow (a gradient algorithm is frequently chosen for reasons of simplicity, robustness and stability).
Moreover, it requires the periodic transmission of a relatively long reference sequence representing a significant loss of usable bit rate.
A particular objective of the invention is to overcome these various drawbacks of the prior art.
To be more precise, one objective of the invention is to provide an equalizer-combiner for diversity receivers offering equalization and combination (weighting) performance better than systems known in themselves.
Accordingly, an objective of the invention is to provide an equalizer-combiner of this kind capable of operating at a very low level. In particular, equalizers of the invention must not be subject to any threshold effect or to any desynchronization.
Another objective of the invention is to provide an equalizer-combiner of this kind which synchronizes the various receive channels in a simple and efficient manner, even on a highly dispersive transmission medium.
Another objective of the invention is to provide an equalizer-combiner of this kind that is capable of eliminating a faulty channel or strongly limiting a channel of poor quality.
Another objective of the invention is to provide an equalizer-combiner of this kind which does not require any significant reduction of usable transmission bit rate.